Aluminum flake pigment

ABSTRACT

An aluminium flake pigment characterized in that the ratio of the average thickness of flakes to the average diameter of flakes is in the range of 0.025 to 0.3, preferably 0.028 to 0.1 and the ratio of the water covering area to the specific surface area is in the range of 0.18 to 0.4, preferably 0.19 to 0.35 is disclosed herein. 
     The aluminium flake pigment of the invention has the improved resistance to the circulation of the metallic paint during the painting.

FIELD OF THE INVENTION

The invention relates to an aluminium flake pigment suitable forincorporating a metallic paint. More particularly, it relates to thealuminium flake pigment having the improved resistance to thecirculation of the metallic paint during the painting.

BACKGROUND OF THE INVENTION

When the paint incorporating the aluminium flake pigment is applied toany article such as car, the paint is necessarily subjected to thecirculation by any of the pumps for preventing the sedimentation of thepigment in the tank. However, the vigorous turbulent flow is caused nearthe pump due to the circulation and therefore the aluminium flakes aresubjected to the excess impact so as to cause the collisions between theflakes and between the flakes and the inner wall of tank, thereby thealuminium flakes being bent and/or ruptured.

If the paint containing the bent flakes is employed, the resultant paintfilm will have the poor metallic brightness because the flakes in thepaint film are oriented irregularly. And, if the paint containing theruptured flakes is employed, the dark paint film will be formed becausethere are a great deal of the torn pieces of the ruptured flakes.

Accordingly, the aluminium flake pigment having the improved resistanceto the circulation is requested.

SUMMARY OF THE INVENTION

The invention provides the aluminium flake pigment which ischaracterized in that the ratio of the average thickness of flakes (d;μm) to the average diameter of flakes (D₅₀ ; μm) is in the range of0.025 to 0.3 and the ratio of the water covering area (WCA; m² /g) tothe specific surface area (SSA; m² /g) is in the range of 0.18 to 0.4.

The aluminium flake pigment of the invention each ratio of which is inthe above range has the improved resistance to the circulation togetherwith the excellent brightness and the high mechanical strength.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is the scanning electron microscopic photograph (×600) showingthe cross section of the painted sheet prepared with the paint in whichthe aluminium flake pigment of Example 1 is incorporated aftercirculation.

FIG. 2 is the scanning electron microscopic photograph (×600) showingthe cross section of the painted sheet prepared with the paint in whichthe aluminium flake pigment of Comparative Example 1 is incorporatedafter circulation.

FIG. 3 is the scanning electron microscopic photograph (×1000) showingthe cross section of the painted sheet prepared with the paint in whichthe aluminium flake pigment of Example 5 is incorporated after thecirculation.

FIG. 4 is the scanning electron microscopic photograph (×1000) showingthe cross section of the painted sheet prepared with the paint in whichthe aluminium flake pigment of Comparative Example 4 is incorporatedafter the circulation.

DETAILED DESCRIPTION OF THE INVENTION

The average thickness of flakes (d), the average diameter of flakesD₅₀), the water covering area (WCA) and the specific surface area (SSA)herein are defined as follows:

The average thickness of flakes (d, μm) is the value obtained bymeasuring the water covering area (WCA) per g of the metal content andthen calculating according to the following equation.

    d (μm)=0.4 (m.sup.2×μm×g.sup.-1)/WCA(m.sup.2×g.sup.-1)

Please refer to J. D. Edwards and R. I. Wray, Aluminium Paint and Powder(3rd Ed.), pp. 16 to 22, Reinhold Publishing Corp., New York(1955).

The average diameter of flakes (D₅₀, μm) is the value obtained bymeasuring with a Microtrac particle-size analyzer.

The water covering area (WCA; m² /g) is the value obtained by measuringaccording to the method described in J. D. Edwards and R. I. Wray,Aluminium Paint and Powder (3rd Ed.), pp. 16 to 22, Reinhold PublishingCorp., New York(1955).

The specific surface area (SSA; m² /g) is the value obtained bycalculating the nitrogen gas volume adsorbed on the flake surface per gof flakes according to BET method.

In the invention, the aluminium flake pigment should have the ratio ofthe average thickness of flakes (d) to the average diameter of flakes(D₅₀) in the range of 0.025 to 0.3. When the flakes have the ratio d/D₅₀of less than 0.025, the flakes will be easily bent and/or ruptured dueto the circulation because they are very thin. The higher the ratiod/D₅₀ is, the lower the percentages of the flakes which are bent and/orruptured due to the circulation are. When the flakes has the ratio d/D₅₀of above 0.3, however, the flakes in the paint film are not regularlyand in parallel oriented because each flake can move freely although theflakes have the high resistance to the circulation, thereby theresultant paint film will have the poor metallic brightness.

Preferably, the aluminium flake pigment in the invention has the ratiod/D₅₀ in the range of 0.028 to 0.1.

Further, the aluminium flake pigment in the invention should have theratio of the water covering area (WCA; m² /g) to the specific surfacearea (SSA; m² /g) in the range of 0.18 to 0.4. As the observation of theflakes having the ratio WCA/SSA in the above range under the scanningelectron microscope, it was found that most of the flakes have therounded profiles and the uniform thickness throughout them and thefinely divided powder or fragments are hardly attached on the flakes.The flakes of such a form are supposed to have higher resistance tocirculation. While, according to the observation of the flakes havingthe ratio WCA/SSA of less than 0.18 under the scanning electronmicroscope, the flakes whose ends are very thinned such as the flakeshaving the corrugated profiles, the partially cracked and/or brokenflakes and the flakes having the finely divided powder and/or brokenfragments on their surfaces were observed, which are supposed to lack inthe resistance to the circulation. The higher the ratio WCA/SSA is, thelower the percentages of the flakes which are bent and/or ruptured dueto the circulation are. When the flakes have the ratio WCA/SSA of above0.4, however, it is not suitable since the productivity of the aluminiumflake pigment is unsatisfactory although the flakes have the highresistance to the circulation.

Preferably, the aluminium flake pigment in the invention has the ratioWCA/SSA in the range of 0.19 to 0.35.

The aluminium pigment of the invention can be particularly suitable forincorporating in paint. The aluminium pigment is mixed with any vehicle,any resin and other conventional additives to prepare the water or oilypaint. Of course, other pigment(s) may be also incorporated in paint.

Further, the aluminium pigment of the invention can be incorporated ininks, adhesives, the resins and the like.

The aluminium flake pigment of the invention is prepared according toany of the known processes for preparing the metal powder pigment.

The aluminium powder used as the starting material is selected among thecommercially available aluminium powder according to the intended tint.If the higher brightness is intended, extracoarse aluminium powder isselected. If the lower brightness, on the other hand, fine aluminiumpowder is selected. The commercially available aluminium powder can beclassified according to roughness into four groups (extracoarse, coarse,medium and fine). The extracoarse aluminium powder has the largeraverage thickness (d) and average diameter (D₅₀)as well as the smallerwater covering area (WCA) and specific surface area (SSA). Conversely,the fine aluminium powder has the smaller average thickness (d) andaverage diameter D₅₀) as well as the larger water covering area (WCA)and specific surface area (SSA).

The aluminium powder is milled according to a dry or wet millingprocess. Preferably, the aluminium powder is ground together with thegrinding medium such as mineral spirit and if necessary other grindingadditives in suitable mill such as ball mill, hammer mill, rod mill andthe like under carefully controlled grinding conditions until thedesired aluminium flakes are obtained. The rotation speed of mill andthe grinding time are varied depending on the desired characteristics offlakes. Generally, the rotation speed of mill is selected in the rangeof 5 to 44 rpm and the grinding time is selected in the range of 1.5 to15 hours. The temperature and pressure are not particularly limited.

If desired, the aluminium flake may be subjected to the standard leafingtreatment.

EXAMPLES

The invention will now be further described by the following,non-limiting examples.

Example 1

Into the ball mill (inner diameter=500 mm, length=180 mm) containing 50kg of steel balls each of which has a diameter of 2 mm, the formulationconsisting of 1.0 kg of atomized aluminium spherical or spheroidalpowder (Alcan MDX-81, ex Alcan Aluminium Ltd.), 5.0 (of mineral spiritand 500 g of oleic acid was charged. Then, the charge in the mill wasground by operating the mill at 38 rpm (corresponding to 64% of thecritical rotation speed) for 3.5 hours.

After the grinding, the slurry in the mill was washed out with mineralspirit and subjected to the vibrating screens of 150, 350 and 400 mesh,successively. The liquid part of the passed-through slurry was removedthrough the pan filter. The resultant cake (NV=85%) was transferred intothe kneader and kneaded for 1 hour so as to obtain the aluminium flakepigment (NV=80%).

Next, a paint was prepared from 34.3 parts by weight of aluminium flakepigment, 160.0 parts by weight of ALMATEX 100 (acrylic resin containing50.0% by weight of dry matter, ex MITSUI TOATSU CHEMICALS, INC.) and33.3 parts by weight of UVAN 20SE-60 (melamine resin containing 60% byweight of dry matter, ex MITSUI TOATSU CHEMICALS, INC.). The viscosityof the paint was adjusted with a mixed solvent of ethyl acetate, butylacetate, xylene, Solvesso # 100 and Cellosolve acetate (mixed ratio byvolume=30:10:30:20:10) so as to be 20 to 21 seconds on Ford cup # 4.

A portion of the above paint was subjected to a circulation (simulationtest). That is, 300 g of the above paint was charged into a vesselprovided with a jacket for circulating the cooling water and stirred bya stirrer with turbine blades for 1 hour.

The painted sheet was prepared employing the paint with or withoutsubjecting to circulation as follows.

The paint was sprayed on polished mild steel sheet by automatic sprayingmachine so that the resultant paint film has a thickness of about 20 μm.Then, a clear coat was formed on this paint film according to procedureswet-on-wet coating followed by baking at 140° C. for 30 minutes.

The procedure of example 1 was repeated except that the grinding timewas 1 hour.

Example 3

The procedure of example 1 was repeated except that as the atomizedaluminium powder AB-5003 (ex TOYO ALUMINIUM K. K.) was used and thegrinding time was 2.5 hours.

And, 19.1 parts by weight of aluminium flake pigment was used in thepreparation of the paint.

Example 4

The procedure of example 1 was repeated except that as the atomizedaluminium powder Alcan MDX-65 (ex Alcan Aluminium Ltd.) was used and thegrinding time was 2.5 hours.

Example 5

The procedure of example 1 was repeated except that as the fine portionatomized aluminium powder Alcan MDX-65 (ex Alcan Aluminium Ltd.) wasused and the grinding time was 5.5 hours.

And, 14.3 parts by weight of aluminium flake pigment was used inpreparation of the paint.

Example 6

The procedure of example 1 was repeated except that as the atomizedaluminium powder an equal mixed powder of fine portion of Alcan MDX-65(ex Alcan Aluminium Ltd.) and AC-5005 (ex. TOYO ALUMINIUM K.K.) was usedand the grinding time was 8 hours.

And, 9.5 parts by weight of aluminium flake pigment was used in thepreparation of the paint.

Comparative Example 1

The extracoarse aluminium powder described in Japanese Pat. ApplicationLaying-Open (KOKAI) No. 49-14358 was used, which is said to have thehighest quality.

Comparative Example 2

The commercially available extracoarse aluminium powder (ex TOYOALUMINIUM K.K.) was used.

Comparative Example 3

The commercially available coarse aluminium powder (ex TOYO ALUMINIUMK.K.) was used.

Comparative Example 4

The commercially available medium aluminium powder (ex TOYO ALUMINIUMK.K.) was used.

Comparative Example 5

The commercially available fine aluminium powder (ex TOYO ALUMINIUMK.K.) was used.

Test Example 1

The aluminium flake pigment (NV=80%) obtained in each Example andComparative Example was subjected to the following tests.

test 1water covering area (WCA)

The aluminium flake pigment was subjected to a leafing treatment byadding 1 g of stearic acid to 1 g of aluminium flake pigment and thenheating with a small volume of xylene. The leafing flakes weredry-powdered with acetone to prepare the sample.

The water covering area of the thus-prepared sample was measuredaccording to the method described in J. D. Edwards and R. I. Wray,Aluminium Paint and Powder (3rd Ed.), pp. 16 to 22, Reinhold PublishingCorp., New York(1955).

test 2average thickness (d)

The average thickness of the above sample was calculated according tothe above-mentioned equation based on the WCA value obtained in test 1.

test 3average diameter D₅₀)

The average diameter of the above sample was determined with a Microtracparticle-size analyzer

test 4specific surface area (SSA)

The aluminium flake pigment was washed with a mixed solvent of ether andacetone and then dry-powdered with acetone to prepare the sample

The specific surface area of the thus-prepared sample was calculatedfrom the adsorbed nitrogen gas volume according to the BET method.

The test results are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                        WCA        d       D.sub.50                                                                             d/D.sub.50                                                                          SSA   WCA/SSA                                 (m.sup.2 /g)                                                                             (μm) (μm)                                                                              (-)   (m.sup.2 /g)                                                                        (-)                                     ______________________________________                                        Example 1                                                                            0.15    2.7     34.9 0.077 0.80  0.188                                 Example 2                                                                            0.10    4.00    16.0 0.250 0.56  0.180                                 Comp.  0.36    0.89    29.8 0.030 2.25  0.160                                 Example 1                                                                     Comp.  0.50    0.80    31.4 0.025 2.90  0.172                                 Example 2                                                                     Example 3                                                                            0.50    0.80    15.5 0.052 1.45  0.345                                 Example 4                                                                            0.80    0.50    15.9 0.031 3.35  0.239                                 Comp.  0.90    0.44    23.9 0.019 3.92  0.230                                 Example 3                                                                     Example 5                                                                            1.20    0.33    11.2 0.029 3.90  0.308                                 Comp.  1.33    0.30    17.6 0.017 4.48  0.297                                 Example 4                                                                     Example 6                                                                            1.70    0.24     8.5 0.028 5.49  0.310                                 Comp.  1.60    0.25    14.0 0.018 6.20  0.258                                 Example 5                                                                     ______________________________________                                    

Test Example 2

The painted sheet prepared in each Example and Comparative Example wassubjected to the following tests.

test 5brightness

The brightness of the painted sheet prepared with the paint beforecirculation was compared with that of the painted sheet prepared withthe paint after circulation.

The brightness was measured according to the method described inJapanese Pat. Application Laying-Open (KOKAI) No.60-3016 by the TM-typegoniophotometer (ex MURAKAMI COLOR RESEARCH LABORATORY), provided thatthe incidence angle was set at 45° and the oblique angle of the paintedsheet was set at 5°.

test 6number of bent flakes

The painted sheets prepared with the paint before and after thecirculation were observed under the scanning electron microscope so asto count the number of the bent flakes per 100 flakes in the paint film.

The test results are shown in Table

                  TABLE 2                                                         ______________________________________                                                brightness     number of bent flakes                                          circulation                                                                             retention                                                                              circulation                                                before after  (%)      before after                                   ______________________________________                                        Example 1 190      180    95     0/100   0/100                                Example 2 160      155    97     0/100   0/100                                Comp. Example 1                                                                         170       94    55     2/100  23/100                                Comp. Example 2                                                                         157       80    51     4/100  30/100                                Example 3 186      184    99     0/100   2/100                                Example 4 179      166    93     0/100   5/100                                Comp. Example 3                                                                         156      126    81     2/100  17/100                                Example 5 153      152    99     0/100   2/100                                Comp. Example 4                                                                         153      103    67     2/100  10/100                                Example 6  98       96    98     0/100   2/100                                Comp. Example 5                                                                          95       83    87     8/100  *                                     ______________________________________                                         *not determined because there were many bent and/or ruptured flakes.     

The scanning electron microscopic photographs showing the cross sectionsof the painted sheets prepared with the paint after the circulation areshown in FIG. 1 to 4. FIG. 1 and FIG. 2 show the painted sheets inExample 1 and Comparative Example 1, respectively (×600). FIG. 3 andFIG. 4 show the printed sheet in Example 5 and Comparative Example 4respectively (×1000).

EFFECT OF THE INVENTION

The aluminium flake pigment of the invention can retain the originalbrightness after subjecting to the circulation.

The bent and/or ruptured flakes are not substantially found in the paintfilm prepared with the paint incorporating the aluminium flake pigmentof the invention.

The above effects are clear from the scanning electron microscopicphotographs shown in Figures. Please note that the aluminium flakes areoriented regularly in the painted sheets shown in FIG. 1 and FIG. 3,while the aluminium flakes are oriented irregularly in the paintedsheets shown in FIG. 2 and FIG. 4.

Accordingly, the aluminium flake pigment of the invention can be said tobe very excellent in the resistance to the circulation.

I claim:
 1. An aluminum flake pigment wherein the ratio of the averagethickness of flakes to the average diameter of flakes is in the range of0.025 to 0.3 and the ratio of the water covering area to the specificsurface area is in the range of 0.18 to 0.4.
 2. The pigment as claimedin claim 1 wherein the ratio of the average thickness of flakes to theaverage diameter of flakes is in the range of 0.028 to 0.1.
 3. Thepigment as claimed in claim 1, wherein the ratio of the water coveringarea to the specific surface area is in the range of 0.19 to 0.35..Iadd.
 4. An aluminum flake pigment, wherein the ratio of the averagethickness of flakes to the average diameter of flakes is in the range of0.025 to 0.3 and the ratio of the water covering area to the specificsurface area is in the range of 0.18 to 0.4, wherein the flakes have asufficiently uniform thickness to orient the flakes regularly in apainted sheet..Iaddend..Iadd.5. An aluminum flake pigment, wherein theratio of the average thickness of flakes to the average diameter offlakes is in the range of 0.025 to 0.3 and the ratio of the watercovering area to the specific surface area is in the range of 0.18 to0.4, which flakes orient regularly in a painted sheet..Iaddend..Iadd.6.An aluminum flake pigment, wherein the ratio of the average thickness offlakes to the average diameter of flakes is in the range of 0.025 to 0.3and the ratio of the water covering area to the specific surface area isin the range of 0.18 to 0.4, the flakes having a uniform thickness andshape which essentially eliminates bent or ruptured flakes caused bypaint circulation..Iaddend..Iadd.7. The aluminum flake pigment of claim4, 5 or 6, wherein after paint circulation the flakes retain at least93% brightness..Iaddend..Iadd.8. The pigment of claim 7, wherein theretained brightness is between about 93% and 99%..Iaddend..Iadd.9. Thealuminum flake pigment of claim 1, wherein the flakes orient regularly,as observed by scanning electron microscope, when applied in a paintfilm made in the following manner:(a) 34.3 parts by weight of thealuminum flake pigment are mixed with 160.0 parts by weight of acrylicresin containing 50.0% by weight of dry matter and 33.3 parts by weightof melamine resin containing 60% by weight of dry matter, to form apaint; (b) viscosity of the paint is adjusted to 20 to 21 seconds onFord cup #4, with a mixed solvent of ethyl acetate, butyl acetate,xylene, aromatic solvent having a typical flashpoint of 49 degrees C.and CH₂ COOCH₂ CH₂ OOC₂ H₅ (mixed ratio by volume=30:10:30:20:10); (c)300 g of the viscosity-adjusted paint is charged into a vessel providedwith a jacket for circulating cooling water and stirred by a stirrerwith turbine blades for 1 hour; (d) the paint is sprayed on polishedmild steel sheet to form a paint film having a thickness of about 20 μm;and (e) the paint film is baked at 140° C. for 30 minutes..Iaddend.